1. Field of the Invention
The present invention relates generally to digital telephone line circuits having improved adaptive echo cancellation capability, and which are used in conjunction with a two-to-four wire hybrid circuit, and specifically to a technique for simplifying the hardware requirements for adaptively generating and varying the digital filter coefficients of a finite impulse response digital filter in the line circuit in accordance with echo conditions to cancel undesired signal return.
2. Description of the Prior Art
The generation of undesirable echoes in the two-to-four wire conversion (hybrid) circuit of a telephone subscriber line/trunk circuit is a well known problem in telephony. Such echoes result from an impedance mismatch in the balancing network of the hybrid when the balance impedance value in the balancing network does not match a perfect line termination impedance under conditions which normally vary with time. In a digital telephone system, such analog echoes are digitized and transmitted from the calling to the called party together with the desired digitized speech, which can result in a "rain barrel" effect. Various digital filtering techniques have been developed and are known in the telephony art for cancelling this echo in which a replica of the undesired echo signal is derived, and then subtracted from the sum of the desired signal and the undesired echo signal prior to transmission. Known echo cancellation techniques suffer from one or more of the following problems: they are complex and costly to implement, they introduce distortion into the speech path, and they involve the processing of complex signals with associated large scale digital multiplication circuitry, which is both complex and expensive.
For example, a digital echo cancellation circuit for satellite communication and long distance telephone circuits is described in Echo Control in Telephone Communication, by H. G. Suyderhoud, et al., COMSAT Laboratories, 1976. This circuit includes a finite impulse response (FIR) digital filter having variable coefficients to generate a replica of the echo signal for echo cancellation. A New Structure for Adaptive Digital Echo Cancellation, of R. D. Gitlin, et. al., Bell Telephone Laboratories, Holmdel, N. J., 1976 describes the use of a pair of FIR digital filters having variable coefficients for eliminating echo by adapting the coefficients to cause the filters to cancel the echo. The additional publications, Echo Cancellation in a Two-Wire Full-Duplex Data Transmission System with Bipolar Encoding, of H. C. van den Elzen, et. al. Philips Research Laboratories, Eindhoven, Netherlands, 1976; and Echo Cancellation Techniques For Full-Duplex Data Transmission on Two-Wire Lines, of D. D. Falconer, et. al., Bell Telephone Laboratories, Holmdel, N. J., 1976, both disclose the use of FIR digital filters having variable coefficients for adaptively cancelling echoes in data transmission systems.
For structural details of known FIR digital filters, of which there are many, reference is made to U.S. Pat. Nos. 3,639,848 of J. Elliott, A Transverse Digital Filter; 3,665,171 of J. Morrow, A Nonrecursive Digital Filter; and 3,928,755 of M. Bellanger, et. al., A Low Pass Nonrecursive Digital Filter.
All of the foregoing articles include or require means associated with the FIR filters for deriving control signals for varying the filter coefficients which includes or requires substantial complex and costly digital circuitry for digitally multiplying data associated with the signal processing data. In contradistinction, and in accordance with the present invention, a novel correlation technique is described for adaptively deriving the filter coefficients whereby nonlinearities are introduced into the transmitted and received signals to enable the correlation circuit to derive an output representative of the variation of the filter coefficients required to define the requisite echo replica produced by the filter without the heretofor required complex digital multipliers. The correlator output adaptively varies the coefficients of the FIR digital filter in accordance with the instantaneous echo conditions of the line circuit to cancel undesired echo return in a telephone subscriber system.